An atomic frequency standard is a device having a resonant system derived from an atomic or molecular specie experiencing a transition between two or more well-defined energy levels of the atom or molecule. Such a system includes an atomic-controlled oscillator whose frequency is controlled by means of a physics package and associated electronics that are devoted at maintaining the assigned output frequency, typically 5 MHz or 10 MHz, on a very long-term, accurate and stable basis. Such a physics package, frequently referred to as a resonator assembly, typically includes a microwave cavity resonator, a filter cell, an absorption cell, a lamp assembly including a vapor discharge lamp, a photodetector, a temperature control means, and at least one magnetic shield surrounding these components. The general operation of such an atomic frequency standard is well known in the art.
The filter cell, absorption cell, and vapor discharge lamp contain a material having atoms which can be excited to Undergo atomic energy level transitions, such as, for example, rubidium atoms. Typically, an r.f. excitation field is applied to the vapor discharge lamp to produce light. The filter cell contains an isotope of rubidium, such as RB-85, which filters out light with a wave length that will stimulate transition of atoms from a particular energy level to any optically excited energy level. The filtered light is then directed through an absorption cell which contains another isotope of rubidium, such as RB-87, and the filtered light energy absorbed by the RB-87 atoms causes a transition of the atoms from one energy level to another optically excited energy level.
In order for there to be sufficient rubidium atoms available to make the appropriate transitions, a heating device is employed to increase the temperature of the rubidium lamp, absorption cell or filter cell, thereby increasing the vapor pressure in the respective device which in turn increases the number of rubidium atoms in the vapor state. For example, a heater may be operated to increase and maintain the temperature within a resonator assembly to about 70.degree.-80.degree. C. to provide vaporous rubidium, and to increase and maintain the temperature of a lamp assembly at about 110.degree. C.
Rubidium atomic frequency standards require precise temperature control of the lamp assembly. Because of the temperature coefficient of the lamp cell, the thermal control must operate within milli degrees centigrade (0.001.degree. C.) per month to achieve the desired aging performance, and must be kept to a few tenths of a degree centigrade over the ambient temperature extremes to achieve the desired temperature coefficient performance.
Although systems have been invented which attempt to provide the necessary temperature control of the lamp assembly, an improved heater controller for use with an atomic frequency standard is desired.